1. Field of the Invention
The present invention relates to a process of assembling a hollow rotor shaft, and more specifically to a small twin spool gas turbine engine that uses the hollow high speed rotor shaft.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
A gas turbine engine is used to power aircraft such as an unmanned aero vehicle (UAV) that includes video reconnaissance aircraft or a cruise missile. For use in a cruise missile, the engine is intended to be used only once and for typically less than one hour of flight time. In a cruise missile, the space to secure the engine and the space to hold the fuel is limited. Improving the fuel efficiency of the engine will allow for less space to store the fuel, or allow for a longer range using the same amount of fuel. For a UAV like a video reconnaissance aircraft, the hover time is the most important factor. A more efficient engine will allow for a longer hover time over the target.
A UAV such as the two described above is typically a small aircraft that requires a power plant of less than 300 pounds thrust, and typically of less than 100 pounds thrust. Up to now, these aircraft have been powered by internal combustion engines—which are low in efficiency—or small single spool gas turbine engines. The small gas turbine engines provide higher fuel efficiency than the internal combustion engines such as the Wankel engine used in the US Army Shadow UAV. A twin spool gas turbine engine is about twice as efficient as a single spool gas turbine engine. However, twin spool gas turbine engines are used in larger thrust producing engines. All present day known twin spool gas turbine engines are much too large to be used in a small UAV such as those described above.
Scaling down a larger twin spool gas turbine engine has significant design problems. In a prior art twin spool gas turbine engine, such as the PW4000 series engine produced by Pratt & Whitney, cannot be scaled down much below the 300 pound thrust range before rotor dynamics problems occur that will cause the rotor shafts to exceed the critical speed. The critical speed is when the third mode falls below the rotor shaft operating speed. The third mode will cause the rotor shaft to self destruct from the infinite-approaching bending stresses that develop from the rotation. Thus, for a small twin spool gas turbine engine to be useful in a small UAV with power under the 300 pound thrust range, a new design is required for the engine.